The fighting is pretty darn simple, though you'll need to check the help at first to figure out the various types of special attacks assigned to the shoulder buttons. All of the main attacks, however, are done with a mere two buttons, with each of the 24 characters having a few attack variations depending on what order you press 'em in. Another button picks stuff up for purposes of throwing. And that's the fighting system: simplistic and shallow, with no especially interesting dynamics. It can be fun to watch the colorful carnage at first, but repetition and tedium soon sets in, as most fights are pretty similar to each other.
Since the actual brawling's not fantastic, Grand Adventure mode is the glue that holds this game together. It's reasonably enjoyable to sail around and encounter dozens of the ridiculous characters from the One Piece universe, each spouting preposterous dialogue as they attack you for no discernable reason. Then you win crap for your galleries, and maybe level up your warrior. Most of the collectible items serve no purpose, especially the 200+ trading cards from the official One Piece card game. You can collect 'em, but can't actually play the game. Oops. Still, the endless flow of unlockable junk might just be enough to keep kids playing.